Sonic testing needed for 0.060 over?

[PolloLoco]

From Wikipedia http://en.wikipedia.org/wiki/AMC_Straight-6_engine

The longer stroke 258 crankshaft and rods will drop right in the slightly larger bore 4.0 L block, easily creating an even higher torque 4.5 L engine with the stock bore (4.6 L bored 0.030 inches over, and 4.7 L with a 0.060 inches overbore). Blocks should be sonic checked for adequate cylinder wall thickness before boring 0.060 inches over.

From some stroker webpage (I'm sure that you've seen it) http://jeep4.0performance.4mg.com/stroker.html

Add 30 thou or 60 thou overbore pistons and displacement is increased to 280ci (4587cc or 4.6L) and 284ci (4657cc or 4.7L) respectively. If you're feeling really brave, you can overbore your engine to 4.00" and use custom forged pistons. The block must be sonic-tested to ensure that there's adequate cylinder wall thickness before overboring this far.

Is sonic testing really necessary for 0.060" over? I can understand a 4.00" bore, but seriously for 3.935" bore? Someone please tell me that it's not needed or that you did it and you still have a solid engine.

[John]

That is "sonic test before boring to a 4" bore.
John

[5-90]

For most engine block castings, +.060" is the accepted maximum overbore without testing (some variation happens - I think the third-gen Hemi castings are good for a max of +.040" or so.)

The sonic checking is to make sure the cores didn't shift when the heavy molten iron hit them - the bores need to be either centred or the cylinder wall is a bit thicker on the major thrust side - more material there is desireable.

So, you're not so much checking for internal casting flaws (they'd probably have manifest by now anyhow,) but you're checking for raw casting thickness. The acceptable minimum for cylinder wall thickness in a cast iron block is taken to be one-tenth of an inch, or .100", per my Engine Machining instructors at De Anza. To bore a 242ci block to 4.000" requires the removal of .0625" of material all around, so you want at least .1625" wall thickness before machining. (By contrast, a +.060" bore means you'll be removing .030" all around, so you're taking a little over twice as much as the accepted maximum.)

[Plechtan]

Most (all?) engine shops have a sonic tester, and can check your block. The problem areas are the front and rear of the cylinders. the left and right sides usually have plenty of meat.